Signal Detection in CamScan Low Vacuum Microscopes

The conventional Thornley-Everhart Secondary Electron detector, which relies on a high voltage supply for its operation, cannot function in a low vacuum.

An alternative devise has to be used and CamScan provide two types of electron detector for operating in these conditions.

Specially designed LEASE - Low Energy Amplified Secondary Detector.
Conventional Back Scattered Electron Detector.

LEASE Detector
The Lease detector obtains images from those electrons that have interacted with the gaseous environment around the sample. During Lease operation the sample and its immediate surroundings are held at a potential that enhances the collection of these low energy electrons which carry information about the sample surface and topography.

The detector is not critically dependent on working distance and can image over a wide range of beam voltages from < 5 to >30 Kv. The column and detectors do not limit the field of view of the microscope and very low magnification images (<10x) can be obtained. There is no problem in operating at the standard analytical position for EDX and WDX indeed working distances greater than 50mm can be used.

Chamber pressures up 200 Pascals can be used but for most applications excellent results will be achieve with a setting of less than 50 Pascals and a resolution <5nm can be achieved at 30 kV and 8mm WD from an appropriate sample.

The microscope resolution will depend on the normal factors of kV, WD, probe current and signal/noise

Back Scattered Electron Detector
A conventional back scattered electron detectors of either the Semiconductor or Scintillator design can be used as complementary detectors.

It has been common practice for many years to use these detectors for low vacuum SEM imaging They provide the additional benefit of both compositional (Z) and topographical contrast information in both High and Low Vacuum modes.

Deben Coolstage Specimen Cooling Unit for Low Vacuum Operation

System description
The Deben Coolstage has been designed for use during low vacuum operation and is a simple temperature controlled specimen holder to control evaporation of water vapour from "wet" samples.

The system comprises a thermally isolated specimen holder with a single stage Peltier device and dual temperature sensor, a vacuum feedthrough flange, water chiller, power supply box and keypad for digital temperature readout and control. Temperature range is -25ºC to +50ºC at 300Pa.
The specimen holder is water cooled from a small self-contained closed loop chiller box located approximately 2m away from the SEM. The temperature of the specimen holder is monitored and controlled by a microprocessor.

A small keypad with bright VF display sets the required temperature and displays target and current temperatures. The specimen holder has been designed to minimise image drift due to temperature change, giving a stable image at high magnification.
An integrated RS-232 interface allows temperature to be set and read from the SEM.

Applications

Why cool specimens in Low Vacuum?

With increased use of Low Vacuum or Variable Pressure microscopes, many microscopists now appreciate the need to control water evaporation from wet samples. By cooling a wet specimen, water evaporation may be slowed or depending on chamber pressure stopped altogether. As the chart indicates, saturated vapour pressure of water decreases considerably with temperature. At room temperature water will very quickly evaporate causing considerable changes to the specimen structure.

At 200Pa, specimen temperature needs to be less than -16ºC and at 85Pa specimen temperature needs to be less than -25ºC to stop water evaporation. Therefore by cooling a specimen to -25ºC, chamber pressure may by 85Pa with no water evaporation from the specimen. The advantages of this procedure are clear to see. By cooling a specimen in a low vacuum, changes in specimen structure due to water evaporation can be minimised and the ability to operate at higher vacuum provides a better signal to noise ratio and clearer images.